Objectives To explore how fat, lean and body mass index (BMI) track in childhood and how this relates to parental obesity.

Design and Setting Prospective population-based cohort study: Avon Longitudinal Study of Parents and Children, UK.

Method Height, weight and leg-to-leg bioelectrical impedance (BIA) were collected at ages 7 and 11 years, as well as pre-pregnancy parental heights and weights. For BMI International Obesity Task Force thresholds of obesity and overweight were used. Impedance data were expressed as separate lean and fat z scores, internally standardised for gender, height and age and a child was defined as over-fat if fat z score was >85th and very over-fat if >95th internal centile.

Results Data were available for 7723 and 7252 children at ages 7 and 11 years, respectively (6066 at both time points). Of those obese at age 7, 75% were still obese at age 11, while of those who had been overweight 16% had become obese and 20% now had normal BMI. Both fat and lean z scores showed moderate levels of tracking (correlation coefficients 0.70 and 0.73, respectively). Children with one or two obese parents had higher fat z scores at age 7 and showed greater increases in fat thereafter. They were more likely to be very over-fat at age 7 and, of these, 69% remained so at age 11 compared to only 45% with non-obese parents (p <0.001).

Conclusions Children of obese parents already have high fat levels at age 7 and are more likely to remain very over-fat.